Evolution of secondary sex ratio

Although a single male can frequently ensure the production of sufficient microgametes to cover the needs of the entire population, the ratio of the number of progeny of the male and female sex, i.e. the secondary sex ratio of the population, is close to one with surprising frequency. The mortality of males and females frequently differs during maturing and finally also in adulthood. Consequently, in adulthood, the tertiary sex ratio can be very different, usually biased in favor of females. In contrast, the primary sex ratio, i.e. the ratio of male and female zygotes immediately following fertilization of the eggs, is, to the contrary, usually biased in favor of the sex whose embryos die more frequently prior to birth. For example, in human beings, there are 160 male zygotes for every 100 female zygotes, while only 106 boys are born for every 100 girl babies (Dorak et al. 2002). It is obvious at first glance that a secondary sex ratio equal to one is not optimal from the standpoint of the population for a great many species. If more females were to be born at the expense of males and every male were to fertilize a greater number of females, the population as a whole could grow faster than when the numbers of males and females are approximately equal.

            This paradox has long drawn the attention of a number of biologists. Consequently, a number of hypotheses have emerged in to explain its existence. Hypotheses considering the same numbers of males and females to be a consequence of a genetic mechanism of determining the sex of the embryos are currently falling into disfavor. A genetic mechanism of determining sex should primarily affect the ratio of the two types of heterogametes and thus the ratio of male and female zygotes immediately after fertilization. This ratio, the primary sex ratio, however, frequently deviates substantially from a value of 1 and, as already mentioned above, approaches a value of 1.6 in favor of male zygotes in humans (Dorak et al. 2002). Similarly, comparative and experimental studies have demonstrated beyond the shadow of a doubt that a genetic mechanism of determining the sex of a zygote is extremely plastic at both the interspecies and intraspecies level. It is known that completely different mechanisms that, together, could theoretically ensure a quite arbitrary ratio of males and females in the progeny, exist in the individual taxa. Simultaneously, it is apparent that a population exposed to a selection pressure for a change in the sex ratio usually reacts quite easily to the given pressure and changes the sex ratio in the appropriate manner (Orzack & Gladstone 1994). It is thus apparent that a secondary sex ratio of 1 is not a consequence of the mechanism employed to determine the sex of the embryo but rather a result of quite specific selection pressures and that it is adaptive.

            A value of the secondary sex ratio equal to one can also be explained by the action of individual selection. The effect of this factor on the secondary sex ratio is expressed in the Shaw-Mohler principle (Shaw & Mohler 1953). Translated from the language of mathematics to normal language, this principle says that, at the instant when, because of the momentary ratio of males and females in the population, it is preferable to produce members of one sex rather than members of the other sex, those individuals, who produce more progeny of momentarily more valuable sex, will be at an advantage.

            Under normal circumstances, a population is in equilibrium in the numbers of males and females. The selection value of males (most readily expressed as the number of progeny that they leave behind) is the same as the selection value of females. Simultaneously, it is not important that all the females in the population have approximately the same number of progeny, while there are frequently enormous differences amongst males in the number of progeny. The variance value has no effect on the selection value of a member of a certain sex, only the value of the average number of progeny per member of that sex is important. If males predominate because of a random fluctuation in the population, then those individuals that, on the basis of their genetic predisposition, produce more progeny of the female sex are at an advantage. Thus, the population gradually returns to equilibrium. The temporary increase in the sex ratio amongst humans in the post-war years has been cited as an example of this phenomenon in the past. However, newer studies have shown that, for example, the men that returned to England from the battlefields of the Ist World War were more than 3 cm taller than those that died. As taller men exhibit a higher sex ratio in their progeny, the increased sex ratio in the post-war years can be fully explained by the higher death rate of shorter men in the military conflicts {13730}.

            It follows from game theory that the optimal strategy for an individual is to invest the same amount of energy into production of sons as into production of daughters. Under conditions where the production of sons is just as costly as the production of daughters, the ratio of young of both sexes in the population settles at a value of one.

            This explanation of maintenance of equal numbers of the two sexes was apparently first proposed by R.A. Fisher in 1933 (Fisher 1958). However, it must be admitted that later mathematical analyses of the relevant model demonstrated that establishment of equilibrium through such individual selection is too slow and that some other mechanisms are apparently also active in a great many species (James 1995).

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more